Electric Vehicles: Battery Technology

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The current battery technology is Lithium-ion. It’s a high density battery that uses relatively cheap materials compared to earlier batteries. They are built into packs that are fixed under the EV. Recharging is still an issue, but new standards for charging points are coming and manufacturers are setting up joint companies to develop common battery units that may be replaced quickly by automated stations.

Battery technology

In our conventional vehicles, the battery is used to start the engine and provide storage for electrical power produced by the alternator. In an EV the main or “traction” battery is used to power the electric motor to move the vehicle and may supply high power equipment such as the interior heater and air con. Some EVs use an additional “leisure” or “auxiliary” battery, which supplies electronic equipment such as electric windows and the stereo system.

The main battery uses lithium-ion technology and it’s made in the same sort of way to the battery we have in our conventional vehicles. It has an anode, cathode and an electrolyte. The anode is made of a carbon material such as graphite. The cathode may be made of different sorts of materials depending on the capacity and power needed. For example, lithium manganese-oxide is popular with EV makers because manganese is not high in price and is readily available. The electrolyte, which lets current flow between the anode and cathode, is made of lithium salts in a solvent.

Lithium-ion battery cells are put together into battery packs along with fuses, maintenance and control circuitry. For example, the main battery pack used in the Nissan Leaf and the Renault ZE EVs, are built-up from 48 modules each containing four lithium-ion cells and gives 360V DC with 24kWh capacity. Another example is the Mitsibushi i-MiEV, which has an 88 cell battery giving 330V DC at 16kWh. The capacity figures of 24kWh and 16kWh mean that the battery is able to supply 24kW or 16kW of power for 1 hour before it’s exhausted and needs to be recharged.

Recharging an electric vehicle battery

Charging a main battery may take hours using a regular domestic supply point. But this is dependent on battery capacity. The higher the battery capacity the longer it will take to charge from a domestic supply. The on-board charger limits the amount of power it takes to about 2.3kW. So to fully charge a 16kWh battery will take: 16 / 2.3 = 6.95 hours or roughly 7 hours. A 24kWh battery takes: 24 / 2.3 = 10.43 or roughly 10 hours 30 minutes.

A high current domestic unit that is able to supply 16A or 32A will charge the battery quicker. It’s wired directly into the consumer unit and uses a special charging connector. The connector is fitted with special control pins to tell the EV it’s connected to a high current supply. In this case the on-board charger limits the power taken to 16A (3.6kW). So to fully charge a 16kWh battery will take: 16 / 3.6 = 4.44 hours or roughly 4 hours and 30 minutes. Similarly a 24kWh battery will charge in: 24 / 3.6 = 6.66 hours or roughly 6 hours and 40 minutes.

Note that some EVs may only be charged at a rate of 16A (3.6kW) so a 32A (7.2kW) installation will not charge the EV any quicker. In the future all EVs will be able to charge at 32A (7.2kW) from domestic units. This will cut recharging time dramatically.

When out and about, there are chargers placed at car parks and shopping centers. Some are the same as charging at home using a 2.3kW domestic supply and some use the higher 16A (3.6kW) supply.

In the future as battery capacities go higher, there will be high voltage charging stations that are able to supply 22kW, 44kW and even 60kW.

Replacing an electric vehicle battery

Another option is simply to swap a discharged battery with a charged one. We would drive into a station over an area that recognizes the type of EV. An automated system would then take out the battery and put in a charged one in about three minutes. This system is ready to be used in Israel and Denmark this year (2011).

This technology needs the EV makers to agree on one or more standard batteries and put in development for simple, automated exchange.

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